UPDATE 1-Sibir eyes up to $100 mln loss from shareholder case

* Says funds diverted could result in $100 mln loss

* Says expects to recover substantial amount of claim

* Confirms acting CEO Stuard Detmer as Chief Executive

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LONDON, April 28 (Reuters) - Sibir Energy (SBE.L: Quote, Profile, Research), the Russian oil and gas explorer that is suing one of its main shareholders, said on Wednesday provisions for the diversion of funds would result in a 2008 loss after exceptionals of up to $100 million.

The London-listed company said investigations into transactions by 23.5 percent shareholder Shalva Chigirinsky were ongoing, but it reaffirmed the sum involved was estimated to total about $400 million.

Sibir said it was negotiating with Chigirinsky, and if a settlement were successfully implemented it would receive a substantial amount of its total claims.

The company said the precise amount of the loss could change as it continued a detailed review of its affairs.